Human transport devices serve to move a person over a surface and may take many different forms. For example, a human transport device, as the term is used herein, may include, but is not limited to, wheelchairs, motorized carts, all-terrain vehicles, bicycles, motorcycles, cars, hovercrafts, and the like. Some types of human transport may include stabilization mechanisms to help ensure that the device does not fall over and injure the user of the transport device.
A typical four-wheeled wheelchair contacts the ground with all four wheels. If the center of gravity of the combination of the wheelchair and the user remains over the area between the wheels, the wheelchair should not tip over. If the center of gravity is located above and outside of the ground contacting members of the transport device, the transport device may become unstable and tip over.
Referring now to FIG. 1A, atypical wheelchair 100 is shown. The wheelchair 100 and the user 102 define a frame. The frame has a center of gravity 104 located at a position vertically disposed above the surface 106. The term “surface” as it is used herein refers to any surface upon which a human transport device may sit or locomote. Examples of a surface include flat ground, an inclined plane such as a ramp, a gravel covered street, and may include a curb which vertically connects two substantially parallel surfaces vertically displaced from one another (e.g., a street curb).
The surface 106 may be at an incline as compared to the horizontal axis 108 (which is a line in the plane transverse to the local vertical). The angle by which the surface 106 is offset from the horizontal axis 108 is called the surface pitch and will be represented by an angle denoted as θs.
The front wheel 112 and the rear wheel 110 of the wheelchair 100 are separated by a distance d. The distance d between the two wheels may be measured as a linear (e.g., straight line) distance. The wheels 110 and 112 typically have opposing counterparts (not shown) on the other side of the wheelchair. The opposing counterparts may each share an axis with wheels 110 and 112, respectively. The area covered by the polygon which connects the points where these four wheels touch the ground (or the outside portions of the ground contacting parts, when the ground contacting part may cover more than a point) provides an area over which the center of gravity 104 may be located while the wheelchair remains stable. This area may be referred to as the footprint of the device. The footprint of a device, as the term is used herein, is defined by the projection of the area between the wheels as projected onto the horizontal plane. If the center of gravity is above this location, the transport device should remain stable.
If the center of gravity 104 is vertically displaced above the surface 106 and outside the footprint (i.e., the projection of area between the wheels 110 and 112 onto the horizontal plane), wheelchair 100 may tip over. This could happen, for example, when the wheelchair is on a surface that has a steep incline, or, alternatively, if the user ‘pops a wheelie’ in order to surmount a curb, for example. When on a steep incline, the center of gravity 104 may shift back and cause the wheelchair 100 to flip over backwards. This is shown in FIG. 1B where the center of gravity 104 is located at a position that is outside the footprint of the wheelchair 100. The center of gravity 104 is shown including a gravity acceleration vector (g) which linearly translates the center of gravity 104 in a downward direction. The wheelchair 100 may rotate about an axis of the rear wheel 110 until the wheelchair 100 contacts the surface being traversed.
User 102 may help to return the center of gravity 104 to a location that is above the area between the, wheels 110 and 112 by leaning forward in the wheelchair 100. Given this limited control of the location of the center of gravity 104, it is clear that human transport devices such as wheelchairs may encounter great difficulties when traversing uneven surfaces such as a curb or steps.
Some vehicles, by virtue of their weight distribution or typical modes of operation are prone to fore-aft instability and end-over-end (“endo”) rollovers. In operation of an all-terrain vehicle (ATV), for example, it is not always possible or desirable to maintain all wheels of the vehicle in contact with the underlying surface at all times. Yet, it is desirable to preclude loss of control of the vehicle or end-over-end roll-over. ATVs may benefit from stabilization in one or more of the fore-aft or left-right planes, especially under conditions in which fewer than a stable complement of wheels are in contact with the ground. Vehicles of this sort may be more efficiently and safely operated employing control modes supplementary to those described in the prior art.